High frequency structure



Sept., 16, 1958 c. E. RICH HIGH FREQUENCY STRUCTURE Original Filed Sept. 22, 1949 INVENT ES E BY MM ATTORNEY Sept. 16, 1958 ci. E. RICH HIGH FREQUENCY- STRUCTURE Original Filed Sept. 22, 1949 2 Sheets-Sheet. 2

INVENTOR ,QL 5 E. /CH

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ATTORNEY United States Patent O HIGH FREQUENCY STRUCTURE Charles E. Rich, Hempstead, N. Y., assignor to Sperry Rand Corporation, a corporation of Delaware Original application September 22, 1949, Serial No. 117,187, now Patent No. 2,687,490, dated August 24, 1954. Divided and this application March 24, 1953, Serial No, 344,280

9 claims. (C1. 315-548) This invention relates to electron discharge devices and more particularly, to an improved high-power klystron tube, and is a divisional application of parent application Serial No. 117,187, filed September 22, 1949, now U. S. Patent 2,687,490, dated August 24, 1954. One of the features of the lclystron tube described in the parent application is the use of novel magnetic focusing means which makes it possible to focus a very much higher density electron beam in a klystron tube than has heretofore 'been achieved. As a result of the novel magnetic focusing means incorporated in the tube structure disclosed, a klystron capable of delivering exceedingl large magnitudes of power is feasible.

However, if the maximum advantages -associated with a high-density electron beam are to be realized, it is advisable to eliminate the use of grids whenever possible. With the removal of grids the loss of beam current through direct interception thereby, and the dissipation of heat developed from such interception, is eliminated.

According to the present invention a gridless Vextremely high-power Vtube structure is provided wherein an 'axially-directed magnetic field extends through the drift tube and consequently confines electrons in these regions to a beam envelope extending parallel to the tube axis. Moreover, the pole pieces associated with the magnetic circuit are included as integral parts of the tube structure and also serve to define a portion of the vacuum envelope of the tube structure.

While the structure of the instant invention has been employed satisfactorily for continuous wave operation,

it has been found particularly suitable for pulsed operation at an operating frequency of the order of 10,000 megacyeles per second yielding a peak power output well in excess of kilowatts. In the case of pulsed operation, using pulses of the order of .5 microseconds, ion focusing is not available to assist in shaping the beam. Moreover, the loss of beam current by'direct interception militates against the use of grids to an ex tent even greater than in the case of continuous wave operation. In all high-powered operation, and particularly in producing high-powered oscillatory pulses, the problem of the dissipation of heat becomes exceedingly important. This is particularly the case in providing oscillations of high amplitude having a wavelength of the order of 3 cm. which necessarily involves physically l small tube parts owing to frequency considerations.

vIn employing magnetic electron-directing means care must be taken to avoid excessive multipactor effect which reduces the gain and power output of the device. The structure of the present invention is particularly suitable for minimizing multipactor effect.

An object `of the present invention is to provide an improved high frequency electron discharge device with magnetic focusing.

A further object is to provide an ultra-high-frequency velocity modulation electron discharge device capable of delivering exceedingly large magnitudes of power.

A still further object is to provide a compact, rugged 2,852,715 Patented Sept. 16, 19,58

r* ICC 2 c and high-powered electron tube employing magnetic focusing.

An additional object .lies in the provision of lan improved frequency controlling device for a cavity resonator. v

Other objects and advantages will become apparent from the specification, taken in connection with the `accompanying drawings, wherein the invention is embodied in concrete form.

In the drawings, p Y Y Fig'. 1 is a view, partially in cross-section, of an electron discharge device of the present invention;`

Fig. 2 is a view, partiallyV in cross-section, taken along line 2-2 of Fig. 1;

Fig. 3 is a view, mainly in section, taken along line 3-3 of Fig. 1;

Fig. 4 is a sectional view of a part of Fig. 1 that is particularly useful for showing the novel frequency controlling arrangement.

Fig. 4a is an enlarged view of a part of Fig. 4;

Fig. 5 is a view, partially in cross-section, taken along line 5 5 of Fig. 4.

Similar characters of reference are employed in all of the'above figures to indicate corresponding parts.

Referring to Figs. l and 2, an evacuated electron discharge device or tube 11 is provided with an external magnetic circuit, such as permanent magnet 12 suitably positioned relative to the tube 11 for providing a mag netic field extending parallel to the longitudinal axis of the tube 11 throughout the central region of the` tube 11. YThe magnet 12 is constituted of blocks 13 of highly magnetic material, such as Alnico V, a steel bar 14, and steel cradles 16. Cradles 16 are formed to cooperate with the exterior walls of tube 11 and partially surround magnetic pole pieces which are included as integral parts of the tube 11. Metallic straps 17 extend around the blocks 13, bar 14, cradles 16 and tube 11 and are each joined by means of a screw 18. The parts constituting tube 11 may be more readily described in connection with Fig. 3. While separate parts have been employed to form the 'external permanent magnet 12 for convenience in fabrication, it is of course understood that a single structure composed of highly magnetic material might be employed. This would have the advantage of a more compact external magnet structure.

Referring to Fig. 3, a metallic block 21, suitably apertured, serves to define a portion of the wall surfaces of4 two electromagnetic field defining means, such as cavity resonators 22, 23. Supported by the block 21 is a hollow tubular member 24, the ends of which also serve to define a portion of the wall surface of the cavity resonators 22, 23. The inner cylindrical surface of tubular member 24 forms the walls of a drift tube or drift space 26. Apertured discs 27, 28 are arranged at opposite ends of the tubular member 24 and spaced therefrom to provide end walls for the resonant cavities 22, 23, respec tively. The shape of the discs 27, 28 and the ends of tubular member 24 is such as to provide each of the resonators 22, 23 with a doubly-reentrant configuration. Annular slots 29, 31 (see Fig. 4) are provided adjacent the resonators 22, 23 respectively, to permit the discs 27, 28 to be suitably bowed, thereby facilitating the pretuning of the resonators 22, 23. The block 21, tubular member 24 and discs 27, 28 arepreferablyV madel of copper or a copper-plated .material to provide highly conductive wall surfaces for the resonators 22, 23.

Gap defining means, such as thin walled sleeves 32, 33 are concentric about the longitudinal axis of the tube 11. The sleeves 32, 33 are preferably made of a material having a high melting point and readily machin-Y able, such as molybdenum. The sleeves may be seen more clearly in Fig. 4. By providing the sleeves 32, 33

with a thin wall, for instance of the order of 0.005 inch, a relatively small surface area is presented by the ends of the sleeves 32, 33 to electrons which may impinge thereon, thus minimizing secondary electron emission.

Coupling means 34, 36 are employed for introducing electromagnetic energy into the cavity resonator 22 and coupling energy from the cavity resonator 23, respectively. Windows 37, 33 are provided in the block Y21 of suitable dimensions to effect an impedance match between the Vresonators 22, 2-3 and the coupling means 34, 36, respectively. Each of the coupling means 34, 36 are shown formed of a rectangular cross-section wave guide 39 provided with cooling fins 41 and an end flange 42 suitable for supporting the Window' frame 43, which in turn contains the glass window 44. Inasmuch as the vacuum envelope of the tube 11 is constituted in part of the glass Window 44, the Vframe 43, and the end flange 42, these parts are joined in vacuum-tight connections. For instance, along the lip 46 of the end flange 42 an atomic hydrogen weld may be employed to join the lip 46 of the end ange 42 to the window frame 43. In order to thermally isolate the glass Window 44 from heat developed during such Welding or other joining operations, the outer portion ofthe frame 43 is formed with annular slots 47, 4S to provide a relatively long heat flow path. In addition, the configuration of the outer portion of the frame 43 serves to mechanically isolate the glass window 44 from jarring of the outer portions of the end flange 42 and frame 43.

Located on either side of the resonators 22, 23 lare magnetic means, such as pole pieces 49, 51,.respectively, made preferably ofsoft iron. It has been found advisable toprovide the pole pieces 49, 51 with substantially planar pole faces 52, 53, respectively. The pole faces 52, 53 are preferably disposed substantially at right angles to the longitudinal axis of the tube 11 in order to obtain a more uniform .magnetic field. The pole pieces 49, S1 are formed with aligned apertures 54, 56, respectively, through which an electron beam may pass. For shielding purposes pole piece 49 is provided with an outer annular extension 57. Pole pieces 49, 51 serve to provide an axially-directed magnetic field with magnetic lines of force extending through the gaps 35, 40 of the resonators 22, 23, respectively, which are defined by the sleeves 32, 33, as Well as through the drift tube 26 when the pole pieces 49, 51 are suitably energized by an external magnetic source, i. e. by the permanent magnet 12 above described.

By applying a positive potential to the pole piece 49 relative to the cathode 58, electrons may be accelerated. Thus, an anode is provided by the pole piece 49 with an anode plane 60 formed by the surface of the pole piece 49 adjacent the cathode 58 and arranged at right angles to the axis of the tube 11.

Cathode 58 is similar to that disclosed in Patent No. 2,564,743 by Chao C. Wang. The cathode 58 is provided with an electron-emitting surface 59, which is spherically-concave about the longitudinal axis of the tube 11. In order to provide rectilinear motion of electronsemanating from the surface 59, field-forming means, such as focusing electrode 6'1, is arranged around the path of the electron beam. The electrode 61 is formed of a Vdisc 62 and a cylinder 63 which are supported by support 67. The disc 62 is centrally-apertured to provide a Erst line conductor or edge 64 spaced from the surface 59 and of greater diameter than the surface 59. The end of cylinder 63 most remote from the surface 59 provides a second line conductor or edge 66, which has a diameter greater than that of edge 64 and is positioned at a greater distance from the surface 59 than the first edge 64. When thesurface 59 is suitably energized by the heater 68 which is connected to battery 69 by means of leads 70, 71, electrons emanate from the cathode. By applying a positive potential to the `pole piece 49 relative to the cathode 58 by means of al suitable source of 4 potential, such as battery 73 and lead 72, electrons emanating from the surface 59 are accelerated towards the anode plane 60. Such 'electrons are electrostatically controlled by the edges 64, 66 provided by disc 62 and cylinder 63, respectively.

The extension 57 of the pole Apiece 49, previously referred to, serves to isolate the cathode 58 and the field forming means, including the edges 64, 66, from the magnetic field.

It has been found preferableto provide an iron skirt 74 surrounding the cathode 58. This skirt 74 is supported by the extension' of the pole piece 49 and prevents the formation of metallic deposits on the inner surface of glass bell 76.V Such deposits might give rise to undesirable arcing and/or disturbance of the electric eld.

The extension 57 of pole piece 49 is joined to the glass bell 76 by means of flanged tubular members 77, 78, which `are connected in a vacuum-tight manner to preserve the internal vacuum of the tube 11.

To collect the electrons after their passage through the resonators, an electron collector 79 is positioned behind the catcherresonator 23. The collector 79 is constituted of a tubular member 81 having a metallic bell 82 at one end thereof, which is formed from a thinwalled cylindrical tubular member, preferably of copper, and pinched at one end after the tube 11 is evacuated, the other end of bell 82 being connected in a vacuumtight manner to the tubular member 81. A further tubular member 33, provided with an end plate 84, is connected to the tubular member 81 and surrounds a portion of the metailic bell 82. Around the tubular membersl 81, 83 are positioned cooling ns S6 which are suitably disposed by Vmeans of spacing rings S7. The tubular member 81 of the collector 79 is connected to the pole piece 51 by apertured disc 83, glass ring 89, and sets of flanged tubular members and 85, with all of theconnections being made in a vacuum-tight manner.

vAn apertured end member 91 is provided at the end of tubular member 81 adjacent the pole piece 51. It has been found advisable to employ a relatively small diameter aperture for the end member 91 in order to prevent the formation of a virtual cathode in this part of the structure.

In operation, a beam of electrons is projected 'successively through the gap 35, the drift tube 26, and the gap 40. While the electron beam is under the influence of the electromagnetic field of resonator 22, the electrons are subjected to velocity variations, resulting in a velocitymodulated beam. In passing through the drift tube 26 the velocity-varied beam becomes grouped or hunched by reason of the faster moving electrons overtaking the slower electrons to'give rise to a density-modulated beam. This density-modulated beam traverses the gap 40 of the output resonator 23 where electromagnetic energy is extracted from the beam to sustain oscillations in this resonator 23. After such extraction of energy the beam passes through the aperture 56 of the pole piece 51 to the electron collector 79.

Electromagnetic energy is introduced into the buncher resonator 22 by Way of coupling means 34. 'Ille input Wave form after amplification is transmitted from the output resonator 23 by Way of coupling means 36.

In the event that pulse operation is desired, the accelerating voltage may be systematically applied to the tube 11. In such case, battery 73 may be replaced by a pulse generator capable of delivering a large negative pulse of suitable magnitude and repetition rate, thereby providing an oscillatory high frequency output pulse through coupling means 36.

ln order to provide an electron beam for projection through the gaps 35, 40 of sufficient density for highpowered operation, ithas been found advisable to employ a cathode emitting surface 59 having considerably greater area than the ultimate cross-sectional area of the electron beam. In the event that the emitting surface 59 was substantially the same area as the electron beam, the emitting surface 59 would be unable to supply the requisite electron density for a suicient length of time to provide a practical and useful cathode 50 for high-powered operation.

As discussed in the above-mentioned Patent No. 2,564,743, rectilinear motion of electrons emanating from a substantially spherically-concave emitting surface may be obtained. By reason of field forming means, such as focusing electrode 61, substantially rectilinear motion of electrons is obtained between the emitting surface 59 to a point almost extending to the anode plane 60. The potential distribution along the boundary of the beam is designed to follow radii of the spherical concave surface 59. Accordingly, along the beam boundary in the cathode region there is no tendency for the electrons to deviate from rectilinear paths owing to the fact that the potential distribution outside the beam is similar to the potential distribution existing inside the beam. Stated somewhat differently, the eld forming means, including the focusing electrode 61, are effective for providing, in the presence of complete space charge, a zero voltage gradient normal to the edge of the electron beam.

Where a smoother grid is included in the anode plane 60, the equipotential lines existing near the anode plane 60 may be maintained more readily as concentric circles. However, when such a smoother grid is omitted, as is the case with the instant structure in order to minimize electron beam interception by such a grid, a special problem is presented by reason of the distortion of the equipotential lines in the vicinity of the anode plane 60. Such distortion tends to cause divergence of the electron beam before it reaches the anode plane 60. As a consequence, the anode aperture 54 is of greater diameter than would ordinarily be required in order to prevent beam interception by the anode or pole piece 49.

The focusing electrode 61, which is electrically connected to the cathode 58 and consequently maintained at the same potential, is effective for shaping the electron beam in the manner indicated substantially through electrostatic means. After the beam passes into the aperture 54 of the pole piece 49, the electrons at some point in their transit are directed in paths which are parallel to the longitudinal axis of the tube 11. It may be considered for design purposes that a parallel electron beam enters the magnetic field at the point A which is located a distance of one radius from the pole face 52. The radius here involved is that of the aperture 54 in the plane of the pole face 52.

In order to simplify the cathode structure 58 it has been found advisable to accomplish the initial shaping of the beam through electrostatic means, taking precautions to prevent magnetic flux from penetrating the cathode 58. The extension 57 of the pole 49 serves to isolate the cathode region from the undesirable magnetic lines of force.

The electrons of the beam in their transit from the plane including the point A through the input gap 35, drift tube 26 and output gap 40 tend to be directed in helical paths of substantially constant diameter within a beam envelope extending parallel to the longitudinal axis of the tube 11 owing to the inuence of magnetic focusing provided by the pole pieces 49, 51 suitably energized by the permanent magnet 12. This condition exists despite the fact that with a high-density beam the space charge forces tending to separate the electrons forming the beam are exceedingly large. Were the magnetic restraining means omitted, a large portion of the beam current would be intercepted by the adjacent structure of the tube 11. With the velocity variations impressed upon the electrons by the electromagnetic eld existing in the resonator 22, the electrons become grouped in the drift tube 26, giving rise to greater electron concentrations. 'Ihe magnetic focusing tends to maintain the beam en- 6 velope at constant diameter prior to and after the formation of the greater electron concentrations, thereby minimizing transverse debunching.

In order to obtain a sutciently high coeicient of coupling to the cavity resonators 22, 23, it is advisable that the diameter of the gaps 35, 40 be slightly greater than the diameter of the beam passing therethrough. In the case of gridless gap coupling, the outer regions of the electron beam will be more tightly coupled to the resonators 22, 23 than the central portion of the beam. In addition, there is less longitudinal debunching in the part of the beam adjacent the Wall of the drift tube 26. There fore, the outer region of the beam manifests a greater degree of modulation than the central region of the beam. In view of the fact that it is the outer portion of the beam that is lost due to transverse debunching and imperfect focusing, extreme care must be taken with regard to these factors.

While the vpercentage of the beam current reaching the collector 79 is an important facts or the operation of the tube 11, the shape of the electron beam is a prime consideration. If the beam is allowed to converge even slightly at any point along the drift tube 26, the debunching forces increase very rapidly with a resultant loss in gain. From a consideration of transverse debunching Which causes a non-magnetically focused beam todiifuse as a square of the drift distance, it is considered good practice from a design standpoint to make the drift distance or length of drift tube 26 less than optimum value. With magnetic focusing the transverse debunching tends to ne suppressed.

In employing an axial magnetic field in order to obtain a suitable configuration for the electron beam care must be taken to prevent mutipactor effect from reducing the gain and output power of the device. With a high-density electron beam, some electrons may deviate from the desired trajectories despite magnetic focusing. When primary electrons impinge upon the surfaces of the resonators 22, 23, secondary electrons may be set free. If the secondary electrons traverse either of the gaps 35 or 40 in a time corresponding to a half-cycle of the radio frequency field maintained therein, they may impinge upon the opposed surfaces of the resonators 22 or 23 to set free additional secondary electrons. These electrons may be in proper phase to move with the radio frequency eld across the gaps 35 or 40 during the next half-cycle. Thus, a condition may arise Wherein a spurious electron cloud fluctuates back and forth across either of the resonator gaps 35 or ttl in phase with the radio frequency energy. Such a condition results in extraction of energy from the field by such electrous.

It has been found that with magnetic focusing multipactor effect becomes more pronounced. A tentative explanation for this condition follows: Ordinarily, in the absence of magnetic focusing, secondary emitted electrons tend to follow the radio frequency eld lines extending through the gaps 35, 40 of the resonators 22, 23. Such iield lines are of unequal length, resulting in secondary electrons following trajectories having different lengths, which minimizes the possibility of resonance occurring. However, with an axially-directed magnetic field, the electrons tendV to follow rectilinear trajectories so that the secondary electrons will tend to move in similar-length paths with similar transit times. Such a condition will more readily give rise to multipactor effect.

Accordingly, in the design of the tube 11, care must be taken to avoid the multipactor effect which seriously limits the gain and output power of the device. It has been found that by the inclusion lof the sleeves 32, 33 for defining the electron-permeable gaps 35, 40 of the resonators 22, 23, multipactor effect is substantially eliminated. With thin walls used for the sleeves 32, 33, as discussed above, the area presented by the ends of the sleeves 32, 33 to electrons deviating from desired paths is relatively small, thereby reducing the magnitude of secondary electron emission from this source.

In addition, it has also been found advisable to employ different internal diameters for each of the sleeves 32, 33 defining each of the resonator gaps'S, at). In other words, the, gap 35 of the buncher resonator 22 may be formed with sleeves having different internal diameters such as shown more clearly in Fig. 4a. For instance, with a device suitabe for sustaining oscillations having a wavelength of the order of 3 cm, and with ultimate beam diameter of approximately 0.084 inch, the sleeves 32, 33 forming the entrance and exit apertures of the gap 35 may be provided wtih internal diameters of 0.190 and 0.125 inch respectively. The sleeves 32, 33 forming the electron permeable gap d of the catcher resonator 23 may be constituted in a similar manner. The diameters of the sleeves 33 are enlarged as aforedescribed for reducing the possibility of interception of beam electrons after transit of the gaps 35 and 49. This further reduces secondary electron emission from the sleeves 33 and further precludes the possibility of undesired multipactor effects occurring in the tube. If desired, all of the sleeves defining the gaps 35, 40 may be provided with different internal diameters.

Referring to Fig. 4, there is shown a novel tuning arrangement for varying the resonant frequency of the resonators 22, 23. Each side wail 92 of the resonators 22, 23 is providedwith a doubly bowed configuration and rigidly attached to the block 93 along two edges thereof. As may be seen more clearly in Fig. 5, the side wall 92 along its other two edges is unconnected. The wall 92, which is preferably made of copper or a copper-plated material to provide a highly conductive surface, is folded back along its middle portion and rigidly connected to a slot in the end of shaft 94. Thus, longitudinal movement of this shaft 94 permits the middle portion of the side wall 92 to move back and forth to alter the resonant frequency of the resonators 22, 2.3.

The shaft 94 is partially surrounded by bellows 96 having an apertured end wall 97 rigidly connected to the shaft 94. One end of the bellows 96 is rigidly connected to the block 93. The connections of the shaft 94, end wall 97, belllows 96 and block 93 are all vacuum tight to preserve the internal vacuum of the tube il. A knurled ring 98 supported by plate 99 and ring 101 is effective for permitting axial movement of the apertured screw portion 102 of the end wall 97. Nuts 103, 164, when suitably soldered to the apertured screw portion 102, limit the axial movement of the shaft 94.

it will be noted that each side wall 92 is rigidly secured to the means including block 93 defining the cavity resonators 22, 23 along two of its edges. rfhe wall 92 along its other edges is spaced from but almost in contact with the cavity resonators 22, 23. By such an arrangement, the wall 92 does not serve to define a portion of the vacuum envelope of the tube 11, other means including bellows 96 positioned beyond wall 92 being effective for performing this function. With the wall 92 partially disconnected from the block 2li, it is possible to subject the wall 92 to greater movement by means of shaft 94 to vary the frequency of resonators 22, 25 over a wider range Without damaging or impairing the functioning of the wall 92 through strain or a permanent set. By partially securing the wall 92 to the resonators 22, 23, spurious fluctuations of the position of the wall 92 owing to vibration or similar causes are minimized. This is particularly advantageous during tuning of the resonators 2, 23, affording a smoother variation of resonant frequency as a function of the angular position of the lrnurled ring 98. Undesirable changes in the values of Q" of the resonators 22, 23 are substantially eliminated.

While wall 92 has been shown secured to block 93, which is preferably made of copper or a copper-plated material to provide highly conductive surfaces, it is apparent that the wall 92, need not be directly connected to the means defining the cavity resonators 22, 23. For instance, such as a connection could be conveniently made to structure located adjacent the means defining the resonators 22, 23. In addition, while each wall 92 has een shown as fastened to the block 93 along two of its edges, it will be understood that a suitable connection could be made at any point of the wall 92.

From the above description it will be recognized that the various objects of the invention have been achieved by providing a klystron tube structure which is rugged andv compact, and yet capable of delivering a high-power microwave output signal. Conventional grid structure is avoided by the use 0f small diameter gap forming elements. This permits the use of a very much higher density electron beam, a beam that would cause a prohibitive amount of heating if intercepted by a grid. The gap forming elements also greatly reduce multipactor effect by presenting a reduced surface area transverse to the path of the electrons from which secondary emission can take place. A contributing factor to the ruggedness of the tube is the novel tuning arrangement described. Vibration and jarring of the tube cannot affect the output frequency.

Since many changes could be made in the above description and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

. What is claimed is:

1. In an evacuated ultra-high-frequency velocity modulation electron discharge device having an axis, a cathode having an electron-emissive surface, a disc having an aperture aligned with and positioned along said axis, a cylinder aligned along said axis and having its inner edge most remote from said cathode disposed in spaced relationship to the aperture of said disc, resonant chamber means having electron-permeable regions aligned along said axis, a portion of said resonant chamber means being formed of a conductive wall, said wall being positionally fixed at a portion of its periphery and free to move alongr another portion of its periphery, whereby said wall is effective for tuning said chamber means without spurious positional variations, drift tube means positioned between said regions and aligned along said axis, two magnetic mutually spaced pole pieces adjacent said chamber means, each of said pole pieces being apertured along said axis and defining a portion of the vacuum envelope of the device, and apermanent magnet having its arms magnetically coupled to said pole pieces respectively at regions thereof remote from the faces of said pole pieces.

2. Apparatus as in claim l wherein said resonant chamber means are provided with an electromagnetic energy coupling means, said coupling means including an electromagnetic energy conductor having a frame at the end thereof remote from said chamber means, said frame having annular slots in its outer region, said slots being effective for providing a relatively long heat flow path in said frame.

3. A high frequency electron discharge tube, comprising means for producing and directing an electron beam along a predetermined axis, a cavity resonator for microwave energy having one apertured conductive portion disposed along said axis in spaced coaxial relationship with a further apertured conductive portion of said resonator with Vthe space between said portions comprising an opening into said resonator, the apertures of said conductive portions being in coaxial relationship with said axis and of suffiicently large diameter for passage of said electron beam through said resonator, first and second open-,ended metallic sleeves enveloping a region of said beam and extending coaxially along said axis from said apertured conductive portions of said resonator, respectively, said sleeves protruding toward each other from said conductive portions and having adjacent annular ends of slightly larger inner diameter than the diameter of said electron beam along said axis at the region of said ends, said annular ends being in closer axially spaced relationship than said conductive portions and defining an annular coupling gap coaxial with said axis in a plane transverse thereto for interaction between said beam and electric lield lines of microwave resonator energy between said annular ends, the surfaces of said adjacent annular ends of said sleeves being of extremely narrow thickness dimensions transverse said axis, said dimensions being appreciably less than the diameter of said beam at said coupling gap with said ends comprising the only spaced metallic surfaces constituting transverse secondary electron emitting areas in the immediate vicinity of said beam and coupling gap to thereby minimize multipactor effect in said resonator.

4. Apparatus as defined in claim 3 wherein said sleeves comprise a high melting point metal.

5. Apparatus as dened in claim 3 wherein the sleeves comprise molybdenum.

6. An electron discharge tube as defined in claim 3, wherein the inner diameter of said annular end of the sleeve farthest from said beam producing means is slightly larger than the inner diameter of said annular end of the sleeve nearest said beam producing means.

7. High frequency apparatus comprising a cavity resonatOr including conductive wall portions having spaced at parallel surfaces separated by a substantially cylindrical surface coaxial with an axis through said parallel surfaces, tuning means in the cavity resonator including a exible member having an appreciable area extending along the axis of said cylindrical surface with opposed parallel linear edges secured to first and second wall portions of said resonator, respectively, for supporting the flexible member within the cavity resonator between said ilat parallel surfaces, and means coupled to said exible member and extending outside the cavity resonator for bending the exible member to vary the resonant frequency of the cavity, said last-named means being radially disposed relative to said axis.

8. A high vfrequency electron discharge tube, comprising means including a spherically concave cathode for producing and directing a high current density electron 10 beam Whose boundary along a rst portion of a predetermined axis coaxial with said cathode is convergent, magnetic means disposed along said axis for providing a strong magnetic field for maintaining a predetermined beam boundary along a second portion of said axis further from said cathode `than said first portion, a cavity resonator for microwave energy having spaced electron permeable apertured conductive wall means in coaxial relationship with said axis for passage of said beam therethrough, the space between said apertured wall means comprising an opening into said resonator, said wall means having coaxially spaced annular edge terminations of slightly larger inner diameter than that of the electron beam thereat, with the space between said edge terminanations comprising an interaction gap for said beam and electrical iield lines of microwave resonator energy, the surfaces of said edge terminations being of extremely narrow thickness dimensions transverse said axis with said dimensions being appreciably less than the diameter ofV said beam at said interaction gap, said edge terminations constituting the only transverse secondary electron emitting areas in the immediate vicinity of said beam and coupling gap to thereby minimize multipactor effect in said resonator.

9. A high frequency electron discharge tube as set forth in claim 8, wherein said apertured wall means of said resonator include'oppositely disposed reentrant tubular portions, said wall means further including oppositely disposed thin-walled annular sleeve members having open ends, each of said sleeve members being supported by a respective one of said reentrant tubular portions with adjacent ends of said sleeve members comprising said annular edge terminations of extremely narrow thickness.

References Cited in the le of this patent UNITED STATES PATENTS 2,304,186 Litton Dec. 8, 1942 2,305,884 Litton Dec. 22, 1942 2,394,396 Mouromtsel et al. s- Feb. 5, 1946 2,403,782 Blumlein July 9, 1946 2,410,109 Schelleng Oct. 29, 1946 2,512,887 Davies et a1 June 27, 1950 2,564,743 kWang Aug. 21, 1951 2,664,908 Varian July 7, 1953 

